Various beers contain many flavor compounds derived from barley malts, hops, yeast fermentation and other raw materials. However, yeast fermentation forms the core of the beer brewing process, as during fermentation the most important flavor compounds are formed. Fermentation is a process in which yeast metabolizes simple sugars in the wort into ethanol and carbon dioxide. However, these components make a relatively minor contribution to the overall beer flavor. The aroma and flavor characteristics of beer result from the minor flavor volatiles produced by yeast during fermentation.
Another important factor is the hop. There are two kinds of hops used in the brewery: bitter hops and aroma hops. Bitter hops are used for lager beers to give extra bitterness to the beer. Aroma hops are used for specialty beers to enhance the flavor.
The use of starter cultures is a common practice in yeast industries. However, a pure yeast strain often does not combine all optimal traits desired in brewing, as there are two major objectives for using specific yeast strains in the beer industry: improving the efficiency of the production process and obtaining a good quality of the final product (Saerens et al. 2010).
An additional, more recent trend in the food and beverage industry is the production of beverages that support human health. For example, brewing industry has invested in research focused on production of beers with less alcohol and sugar.
To ensure a good quality of beer, the reduction of “bad” flavors and the enhancement of desirable flavors have been an important issue in the brewing industry. Reduction of bad flavors is focused on a low production of diacetyl, an undesirable flavor compound in beer with a ‘butterscotch’ aroma. The most desirable flavor compounds in beer are esters and higher alcohols, giving a fruity aroma to the beer (Verstrepen et al. 2003). The most important flavor-active esters in beer are acetate esters, such as ethyl acetate (“solvent”-like aroma) and isoamyl acetate (“banana” aroma), and ethyl esters, such as ethyl hexanoate and octanoate (“apple” aroma). The most important higher alcohol is isoamyl alcohol (“banana” aroma).
Due to the increasing demand for healthier food and beverages, the reduction of ethanol and carbohydrates in alcoholic beverages, especially beer and wine, is of considerable commercial interest. Current production methods of low-alcohol, reduced-alcohol and non-alcohol beer, i.e. modified fermentation or post-fermentation removal of ethanol, result in either a wort-like taste or a loss of aroma components, respectively (Zufall and Wackerbauer 2000). In an alcohol-free lager beer, the absence of ethanol (les than 0.1%) strengthens any “worty” off-flavors in beers produced by a cold contact process.
An improvement of the organoleptic quality of beverages and the development of new beverages can be attained through bioflavoring (Vanderhaegen et al. 2003). This technique relies on the production and conversion of flavor compounds and flavor precursors by biological methods, such as the use of special yeast strains. Traditionally, brewers have distinguished two types of brewer's yeast: ale and lager yeast, according to their use for the production of ales and lagers, respectively. Ale and lager brewer's yeasts belong to the genus Saccharomyces: S. cerevisiae (ale yeast) and S. pastorianus (lager yeast). Lager yeast is used for the production of pilsner beers, which comprises 90% of the beers produced in the world. The other 10% are specialty beers, produced by ale yeast. In Belgium (and now also in USA) one other yeast species is used for the production of Iambic and gueuze beers, Brettanomyces bruxellensis (Verachtert et al. 1989). This is in huge contrast to the wine industry, where a lot of non-Saccharomyces strains are used to enhance the ‘wild’ character of spontaneous fermentations (Domizio et al. 2011). These strains are today commercially available for the wine industry through companies such as Christian Hansen and Lallemand.
International patent application WO 2009/110807 is related to yeast strains for use in fermentation processes and to a method of enhancing flavor in a product of fermentation by use of a non-Saccharomyces yeast strain. WO 2009/110807 relates to fermentation of wine and does not mention use of non-Saccharomyces species in brewing of beer.
No studies have examined the effects of non-Saccharomyces species on the level of esters and higher alcohols in beer. Also only a few studies report on the influence of yeast on hop flavor in beer. Three studies so far report the influence of either Saccharomyces species or Brettanomyces species on the enhancement of hop terpenoids in beer (King and Dickinson, 2003, Daenen et al. 2007 and Takoi et al. 2010).
International patent application WO 2008/077986 describes a process for producing alcoholic beverages with increased and/or different aromatic terpene content by using genetically modified microorganisms expressing genes which code for monoterpene synthase during fermentation. In this WO 2008/077986 Pichia is mentioned in a list of theoretically possible yeast strains (together with other yeast strains such as e.g. Saccharomyces) for fermenting alcoholic beverages (beer is mentioned as an example together with other alcoholic beverages such as wine, cava, champagne, cider and sake). In other words use of Pichia spp. for making of beer is not exactly and unambiguously disclosed in WO 2008/077986.
German patent DD 288619 A5 describes the use of Pichia yeast to ferment beer wort. The beer wort is not added hops.
An understanding of the biological processes that control the presence and amount of compounds in a fermentation process is highly desirable. In particular, being able to regulate the amount and type of acetate esters in beer would be very beneficial to brewers. Regulation of the amount of esters and higher alcohols, and in particular isoamyl acetate, in combination with hop flavors in beer would allow for the development of new technologies permitting the brewers to more precisely alter the amounts of these desirable flavors in their product. Such a technology would, therefore, be of significant commercial value. In addition to that, flavor enhancement can be a useful way to produce low-alcohol, reduced-alcohol or non-alcohol beers. The problem with production of low-alcohol, reduced-alcohol or non-alcohol beers is the wort-like taste or loss of aroma compounds, either because of removal of the ethanol or because of the low density of the wort.
Thus, there exists a need for improved processes for enhancement of desirable flavors in beer.